Jefferson Sexual dimorphism and development in Phocoenoides dalli 



129 



widely (Perrin 1984, Kasuya and Shiraga 1985)1 how- 

 ever the same general trends seem to be apparent in 

 photographs of animals from other populations. Only 

 future studies of these features in other P. dalli popula- 

 tions will reveal if this assumption is justified. 



Sexual dimorphism, testis weight, 

 and mating system 



The moderately extreme sexual dimorphism apparent 

 in this species deserves some discussion. Not only are 

 males significantly longer and heavier than females 

 (Kasuya 1978, Morejohn 1979, Newby 1982), but sev- 

 eral morphological features are exaggerated in adult 

 males. The mating system of Ball's porpoise is un- 

 known, but sexual dimorphism among mammals, in 

 which the male is larger, is usually related to intra- 

 sexuai competition for females in a polygynous mating 

 system. In fact, Ralls (1977) determined that extreme 

 sexual dimorphism is a very good predictor of extreme 

 polygyny. 



Newby (1982) implied a polygynous system for P. 

 dalli, and Landino (1985) suggested a unimale system 

 (either polygyny or monogamy). Landino's prediction 

 is based on the finding that the relative testes weights 

 of various primate species are related to their breeding 

 systems (Short 1979, Harcourt et al. 1981, Harvey and 

 Harcourt 1984). Males with relatively large testes tend 

 to have multimale systems (mostly promiscuity), while 

 those with smaller testes have unimale systems. This 

 is thought to be related to the need to deliver large 

 loads of sperm in promiscuous species, in which the 

 potential for sperm competition exists (see Trivers 1985 

 for a general description of sperm competition theory). 

 Kenagj' and Trombulak (1986) raised the question of 

 whether mating system/testis size predictions apply to 

 cetaceans, but Brownell and Ralls (1986) found sperm- 

 competition theory to be quite useful in explaining 

 mating systems in mysticete cetaceans. 



Kenagy and Trombulak (1986) compared relative 

 testis weights for 133 species of mammals, and found 

 the harbor porpoise Phocoena. phocoena, a closely 

 related species, to have the highest value. This evidence 

 points to extreme multimale (probably promiscuous) 

 breeding for this species, which is consistent with mor- 

 phological evidence. Harbor porpoises show reverse 

 sexual dimorphism, with females larger than males 

 (Gaskin et al. 1984), and males do not appear to possess 

 any secondary sexual characteristics. 



Unfortunately, Kenagy and Trombulak (1986) did not 

 include data on testis weight in Dall's porpoise. In order 

 to compare Dall's porpoise with their information on 

 other species, data on nine adult males presented by 

 Subramanian et al. (1986, 1987) were used in a manner 

 consistent with that of Kenagy and Trombulak. Only 



sexually mature males taken during the season of male 

 sexual activity were used (August-September; Jones 

 et al. 1988). When plotted on Figure 1 of Kenagy and 

 Trombulak, the Ball's porpoise data point fell close to 

 the regression line for all 133 mammalian species, and 

 well below that of the other five species of small odon- 

 tocete plotted. The testes of P. dalli comprise only 

 about 0.22% of their body weight, as opposed to 4.00% 

 in P. phocoena. 



Small testes relative to body weight suggests a 

 single-male system with low copulatory frequency 

 (either monogamy or polygyny) for Ball's porpoise. 

 Monogamy is rare in mammals (<3% of all mammal 

 species are monogamous), and it is generally associated 

 with sexual monomorphism (Kleiman 1977). Thus it is 

 unlikely to be the case for P. dalli. This leaves polygy- 

 ny, and this conclusion is further supported by the sex- 

 ual dimorphism in size (males larger) and the striking 

 secondary sexual characteristics of males in this spe- 

 cies. Although some adult males are present, Kasuya 

 and Jones (1984) and Kasuya and Ogi (1987) reported 

 that the majority of mature males are segregated from 

 mating areas during the mating season in the western 

 Pacific. This finding is also more consistent with 

 polygyny than with monogamy. 



Several species of odontocete cetaceans show sexual 

 dimoi-phism in these same charcteristics. For instance, 

 extremely canted dorsal fins are seen in adult male 

 spinner dolphins, especially those of the eastern form 

 of the eastern tropical Pacific (Perrin 1972, 1975). 

 Killer whales and spectacled porpoises Australopho- 

 caena dioptrica have a significant degree of sexual 

 dimorphism in both size and shape of the dorsal fin 

 (Fraser 1968, Brownell 1975, Heimlich-Boran 1986, 

 Leatherwood et al. 1982). Adult male long-finned pilot 

 whales Globicephala melas have more rounded fins, 

 with thicker leading edges than females and young 

 (Sergeant 1962). 



Postanal humps appear to be common in many spe- 

 cies of odontocetes, although they have been properly 

 described and correlated with age and sex in only a few 

 (published photographs show them in several genera, 

 including Peponocephala, Lagenorhynckus, and Lage- 

 nodelphis). Rough-toothed dolphin S^eno bredanensis. 

 spotted dolphin Stenella attenuata. spinner dolphin, 

 and some common dolphin Delphinus delphis adult 

 males exhibit prominent humps (Norris 1967; Perrin 

 1972, 1975; Evans 1975; Leatherwood et al. 1982). 

 Again, this feature is most exaggerated in eastern 

 spinners. 



Deepened caudal peduncles (not including the post- 

 anal hump) are apparent in photographs of adult male 

 spinner dolphins (Perrin 1972), and Norris et al. (1985) 

 were sometimes able to use this feature to distinguish 

 adult male Hawaiian spinners in the field. To my 



